The current mainstream light sources for illumination are fluorescent lights and incandescent light bulbs; however, compared to fluorescent lights and the like, those using an LED (light-emitting diode) as a light source have small power consumption, longer life span, are excellent on the aspect of safety such as not being hot to the touch, moreover, do not contain toxic substances such as mercury and are also excellent on the aspects of environment, such that they are anticipated to become main stream light sources for illumination in the near future.
White LEDs currently in use are constituted by combining a blue LED and a YAG:Ce (yellow emitting phosphor), and bear the problem that they have poor color rendering properties to demonstrate natural color generation ability, such that, particularly when a red-colored object or a person's skin is illuminated with such white LEDs currently in use, colors illuminated by natural light cannot be reproduced. Thus, as a method for improving the color rendering properties of such white LEDs currently in use, constituting a white LED by combining a near-UV LED and 3 species of emitting phosphors, red, green and blue, or by combining a blue LED and two species of emitting phosphors, red and green, is being examined, and SrGa2S4:Eu has been described as a green emitting phosphor to be used for such purposes (refer to Patent References 1, 2 and 3).
[Patent Reference 1] Japanese Patent Application Laid-open No. 2002-060747
[Patent Reference 2] Japanese Patent Application Laid-open No. 2007-056267
[Patent Reference 3] Japanese Patent Application Laid-open No. 2007-214579